Fix suggestions for `&a: T` parameters

Previously we were suggesting stuff like `fn f( &u32) {}`

11 files changed, 449 insertions, 60 deletions

diff --git a/src/test/ui/destructure-trait-ref.stderr b/src/test/ui/destructure-trait-ref.stderr
index 302917ca02e..1291517928e 100644
--- a/src/test/ui/destructure-trait-ref.stderr
+++ b/src/test/ui/destructure-trait-ref.stderr
@@ -23,10 +23,14 @@ LL |     let &&x = &1isize as &dyn T;
    |          ^^   ----------------- this expression has type `&dyn T`
    |          |
    |          expected trait object `dyn T`, found reference
-   |          help: you can probably remove the explicit borrow: `x`
    |
    = note: expected trait object `dyn T`
                  found reference `&_`
+help: consider removing `&` from the pattern
+   |
+LL -     let &&x = &1isize as &dyn T;
+LL +     let &x = &1isize as &dyn T;
+   | 
 
 error[E0308]: mismatched types
   --> $DIR/destructure-trait-ref.rs:36:11
@@ -35,10 +39,14 @@ LL |     let &&&x = &(&1isize as &dyn T);
    |           ^^   -------------------- this expression has type `&&dyn T`
    |           |
    |           expected trait object `dyn T`, found reference
-   |           help: you can probably remove the explicit borrow: `x`
    |
    = note: expected trait object `dyn T`
                  found reference `&_`
+help: consider removing `&` from the pattern
+   |
+LL -     let &&&x = &(&1isize as &dyn T);
+LL +     let &&x = &(&1isize as &dyn T);
+   | 
 
 error[E0308]: mismatched types
   --> $DIR/destructure-trait-ref.rs:40:13
diff --git a/src/test/ui/mismatched_types/issue-38371-unfixable.rs b/src/test/ui/mismatched_types/issue-38371-unfixable.rs
new file mode 100644
index 00000000000..c4316bfdd18
--- /dev/null
+++ b/src/test/ui/mismatched_types/issue-38371-unfixable.rs
@@ -0,0 +1,5 @@
+fn ugh(&[bar]: &u32) {} //~ ERROR expected an array or slice
+
+fn bgh(&&bar: u32) {} //~ ERROR mismatched types
+
+fn main() {}
diff --git a/src/test/ui/mismatched_types/issue-38371-unfixable.stderr b/src/test/ui/mismatched_types/issue-38371-unfixable.stderr
new file mode 100644
index 00000000000..3c5e765abfb
--- /dev/null
+++ b/src/test/ui/mismatched_types/issue-38371-unfixable.stderr
@@ -0,0 +1,21 @@
+error[E0529]: expected an array or slice, found `u32`
+  --> $DIR/issue-38371-unfixable.rs:1:9
+   |
+LL | fn ugh(&[bar]: &u32) {}
+   |         ^^^^^ pattern cannot match with input type `u32`
+
+error[E0308]: mismatched types
+  --> $DIR/issue-38371-unfixable.rs:3:8
+   |
+LL | fn bgh(&&bar: u32) {}
+   |        ^^^^^  --- expected due to this
+   |        |
+   |        expected `u32`, found reference
+   |
+   = note:   expected type `u32`
+           found reference `&_`
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0308, E0529.
+For more information about an error, try `rustc --explain E0308`.
diff --git a/src/test/ui/mismatched_types/issue-38371.fixed b/src/test/ui/mismatched_types/issue-38371.fixed
new file mode 100644
index 00000000000..0e20835bef0
--- /dev/null
+++ b/src/test/ui/mismatched_types/issue-38371.fixed
@@ -0,0 +1,18 @@
+// run-rustfix
+// see also issue-38371-unfixable.rs
+#![allow(dead_code)]
+
+#[derive(Copy, Clone)]
+struct Foo {}
+
+fn foo(_a: &Foo) {} //~ ERROR mismatched types
+
+fn bar(_a: Foo) {}
+
+fn qux(_a: &Foo) {}
+
+fn zar(&_a: &Foo) {}
+
+fn agh(&_a: &u32) {} //~ ERROR mismatched types
+
+fn main() {}
diff --git a/src/test/ui/mismatched_types/issue-38371.rs b/src/test/ui/mismatched_types/issue-38371.rs
index d2cd2b88ab7..fb9e4c173e7 100644
--- a/src/test/ui/mismatched_types/issue-38371.rs
+++ b/src/test/ui/mismatched_types/issue-38371.rs
@@ -1,27 +1,18 @@
-struct Foo {
-}
+// run-rustfix
+// see also issue-38371-unfixable.rs
+#![allow(dead_code)]
 
-fn foo(&foo: Foo) { //~ ERROR mismatched types
-}
+#[derive(Copy, Clone)]
+struct Foo {}
 
-fn bar(foo: Foo) {
-}
+fn foo(&_a: Foo) {} //~ ERROR mismatched types
 
-fn qux(foo: &Foo) {
-}
+fn bar(_a: Foo) {}
 
-fn zar(&foo: &Foo) {
-}
+fn qux(_a: &Foo) {}
 
-// The somewhat unexpected help message in this case is courtesy of
-// match_default_bindings.
-fn agh(&&bar: &u32) { //~ ERROR mismatched types
-}
+fn zar(&_a: &Foo) {}
 
-fn bgh(&&bar: u32) { //~ ERROR mismatched types
-}
-
-fn ugh(&[bar]: &u32) { //~ ERROR expected an array or slice
-}
+fn agh(&&_a: &u32) {} //~ ERROR mismatched types
 
 fn main() {}
diff --git a/src/test/ui/mismatched_types/issue-38371.stderr b/src/test/ui/mismatched_types/issue-38371.stderr
index 5d2ce9302ec..5a0146dfd21 100644
--- a/src/test/ui/mismatched_types/issue-38371.stderr
+++ b/src/test/ui/mismatched_types/issue-38371.stderr
@@ -1,46 +1,35 @@
 error[E0308]: mismatched types
-  --> $DIR/issue-38371.rs:4:8
+  --> $DIR/issue-38371.rs:8:8
    |
-LL | fn foo(&foo: Foo) {
-   |        ^^^^-----
-   |        |     |
-   |        |     expected due to this
+LL | fn foo(&_a: Foo) {}
+   |        ^^^  --- expected due to this
+   |        |
    |        expected struct `Foo`, found reference
-   |        help: did you mean `foo`: `&Foo`
    |
    = note: expected struct `Foo`
            found reference `&_`
+help: to take parameter by ref, move `&` to the type
+   |
+LL - fn foo(&_a: Foo) {}
+LL + fn foo(_a: &Foo) {}
+   | 
 
 error[E0308]: mismatched types
-  --> $DIR/issue-38371.rs:18:9
+  --> $DIR/issue-38371.rs:16:9
    |
-LL | fn agh(&&bar: &u32) {
-   |         ^^^^  ---- expected due to this
+LL | fn agh(&&_a: &u32) {}
+   |         ^^^  ---- expected due to this
    |         |
    |         expected `u32`, found reference
-   |         help: you can probably remove the explicit borrow: `bar`
-   |
-   = note:   expected type `u32`
-           found reference `&_`
-
-error[E0308]: mismatched types
-  --> $DIR/issue-38371.rs:21:8
-   |
-LL | fn bgh(&&bar: u32) {
-   |        ^^^^^  --- expected due to this
-   |        |
-   |        expected `u32`, found reference
    |
    = note:   expected type `u32`
            found reference `&_`
-
-error[E0529]: expected an array or slice, found `u32`
-  --> $DIR/issue-38371.rs:24:9
+help: consider removing `&` from the pattern
    |
-LL | fn ugh(&[bar]: &u32) {
-   |         ^^^^^ pattern cannot match with input type `u32`
+LL - fn agh(&&_a: &u32) {}
+LL + fn agh(&_a: &u32) {}
+   | 
 
-error: aborting due to 4 previous errors
+error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0308, E0529.
-For more information about an error, try `rustc --explain E0308`.
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/mismatched_types/ref-pat-suggestions.fixed b/src/test/ui/mismatched_types/ref-pat-suggestions.fixed
new file mode 100644
index 00000000000..e9472b51943
--- /dev/null
+++ b/src/test/ui/mismatched_types/ref-pat-suggestions.fixed
@@ -0,0 +1,24 @@
+// run-rustfix
+
+fn _f0(_a: &u32) {} //~ ERROR mismatched types
+fn _f1(_a: &mut u32) {} //~ ERROR mismatched types
+fn _f2(&_a: &u32) {} //~ ERROR mismatched types
+fn _f3(&mut _a: &mut u32) {} //~ ERROR mismatched types
+fn _f4(&_a: &u32) {} //~ ERROR mismatched types
+fn _f5(&mut _a: &mut u32) {} //~ ERROR mismatched types
+
+fn main() {
+    let _: fn(u32) = |_a| (); //~ ERROR mismatched types
+    let _: fn(u32) = |_a| (); //~ ERROR mismatched types
+    let _: fn(&u32) = |&_a| (); //~ ERROR mismatched types
+    let _: fn(&mut u32) = |&mut _a| (); //~ ERROR mismatched types
+    let _: fn(&u32) = |&_a| (); //~ ERROR mismatched types
+    let _: fn(&mut u32) = |&mut _a| (); //~ ERROR mismatched types   
+
+    let _ = |_a: &u32| (); //~ ERROR mismatched types
+    let _ = |_a: &mut u32| (); //~ ERROR mismatched types
+    let _ = |&_a: &u32| (); //~ ERROR mismatched types
+    let _ = |&mut _a: &mut u32| (); //~ ERROR mismatched types
+    let _ = |&_a: &u32| (); //~ ERROR mismatched types
+    let _ = |&mut _a: &mut u32| (); //~ ERROR mismatched types
+}
diff --git a/src/test/ui/mismatched_types/ref-pat-suggestions.rs b/src/test/ui/mismatched_types/ref-pat-suggestions.rs
new file mode 100644
index 00000000000..49d88d13e01
--- /dev/null
+++ b/src/test/ui/mismatched_types/ref-pat-suggestions.rs
@@ -0,0 +1,24 @@
+// run-rustfix
+
+fn _f0(&_a: u32) {} //~ ERROR mismatched types
+fn _f1(&mut _a: u32) {} //~ ERROR mismatched types
+fn _f2(&&_a: &u32) {} //~ ERROR mismatched types
+fn _f3(&mut &_a: &mut u32) {} //~ ERROR mismatched types
+fn _f4(&&mut _a: &u32) {} //~ ERROR mismatched types
+fn _f5(&mut &mut _a: &mut u32) {} //~ ERROR mismatched types
+
+fn main() {
+    let _: fn(u32) = |&_a| (); //~ ERROR mismatched types
+    let _: fn(u32) = |&mut _a| (); //~ ERROR mismatched types
+    let _: fn(&u32) = |&&_a| (); //~ ERROR mismatched types
+    let _: fn(&mut u32) = |&mut &_a| (); //~ ERROR mismatched types
+    let _: fn(&u32) = |&&mut _a| (); //~ ERROR mismatched types
+    let _: fn(&mut u32) = |&mut &mut _a| (); //~ ERROR mismatched types   
+
+    let _ = |&_a: u32| (); //~ ERROR mismatched types
+    let _ = |&mut _a: u32| (); //~ ERROR mismatched types
+    let _ = |&&_a: &u32| (); //~ ERROR mismatched types
+    let _ = |&mut &_a: &mut u32| (); //~ ERROR mismatched types
+    let _ = |&&mut _a: &u32| (); //~ ERROR mismatched types
+    let _ = |&mut &mut _a: &mut u32| (); //~ ERROR mismatched types
+}
diff --git a/src/test/ui/mismatched_types/ref-pat-suggestions.stderr b/src/test/ui/mismatched_types/ref-pat-suggestions.stderr
new file mode 100644
index 00000000000..6ce1f9602dd
--- /dev/null
+++ b/src/test/ui/mismatched_types/ref-pat-suggestions.stderr
@@ -0,0 +1,297 @@
+error[E0308]: mismatched types
+  --> $DIR/ref-pat-suggestions.rs:3:8
+   |
+LL | fn _f0(&_a: u32) {}
+   |        ^^^  --- expected due to this
+   |        |
+   |        expected `u32`, found reference
+   |
+   = note:   expected type `u32`
+           found reference `&_`
+help: to take parameter by ref, move `&` to the type
+   |
+LL - fn _f0(&_a: u32) {}
+LL + fn _f0(_a: &u32) {}
+   | 
+
+error[E0308]: mismatched types
+  --> $DIR/ref-pat-suggestions.rs:4:8
+   |
+LL | fn _f1(&mut _a: u32) {}
+   |        ^^^^^^^  --- expected due to this
+   |        |
+   |        expected `u32`, found `&mut _`
+   |
+   = note:           expected type `u32`
+           found mutable reference `&mut _`
+help: to take parameter by ref, move `&mut` to the type
+   |
+LL - fn _f1(&mut _a: u32) {}
+LL + fn _f1(_a: &mut u32) {}
+   | 
+
+error[E0308]: mismatched types
+  --> $DIR/ref-pat-suggestions.rs:5:9
+   |
+LL | fn _f2(&&_a: &u32) {}
+   |         ^^^  ---- expected due to this
+   |         |
+   |         expected `u32`, found reference
+   |
+   = note:   expected type `u32`
+           found reference `&_`
+help: consider removing `&` from the pattern
+   |
+LL - fn _f2(&&_a: &u32) {}
+LL + fn _f2(&_a: &u32) {}
+   | 
+
+error[E0308]: mismatched types
+  --> $DIR/ref-pat-suggestions.rs:6:13
+   |
+LL | fn _f3(&mut &_a: &mut u32) {}
+   |             ^^^  -------- expected due to this
+   |             |
+   |             expected `u32`, found reference
+   |
+   = note:   expected type `u32`
+           found reference `&_`
+help: consider removing `&` from the pattern
+   |
+LL - fn _f3(&mut &_a: &mut u32) {}
+LL + fn _f3(&mut _a: &mut u32) {}
+   | 
+
+error[E0308]: mismatched types
+  --> $DIR/ref-pat-suggestions.rs:7:9
+   |
+LL | fn _f4(&&mut _a: &u32) {}
+   |         ^^^^^^^  ---- expected due to this
+   |         |
+   |         expected `u32`, found `&mut _`
+   |
+   = note:           expected type `u32`
+           found mutable reference `&mut _`
+help: consider removing `&mut` from the pattern
+   |
+LL - fn _f4(&&mut _a: &u32) {}
+LL + fn _f4(&_a: &u32) {}
+   | 
+
+error[E0308]: mismatched types
+  --> $DIR/ref-pat-suggestions.rs:8:13
+   |
+LL | fn _f5(&mut &mut _a: &mut u32) {}
+   |             ^^^^^^^  -------- expected due to this
+   |             |
+   |             expected `u32`, found `&mut _`
+   |
+   = note:           expected type `u32`
+           found mutable reference `&mut _`
+help: consider removing `&mut` from the pattern
+   |
+LL - fn _f5(&mut &mut _a: &mut u32) {}
+LL + fn _f5(&mut _a: &mut u32) {}
+   | 
+
+error[E0308]: mismatched types
+  --> $DIR/ref-pat-suggestions.rs:11:23
+   |
+LL |     let _: fn(u32) = |&_a| ();
+   |                       ^--
+   |                       ||
+   |                       |expected due to this
+   |                       expected `u32`, found reference
+   |
+   = note:   expected type `u32`
+           found reference `&_`
+help: consider removing `&` from the pattern
+   |
+LL -     let _: fn(u32) = |&_a| ();
+LL +     let _: fn(u32) = |_a| ();
+   | 
+
+error[E0308]: mismatched types
+  --> $DIR/ref-pat-suggestions.rs:12:23
+   |
+LL |     let _: fn(u32) = |&mut _a| ();
+   |                       ^^^^^--
+   |                       |    |
+   |                       |    expected due to this
+   |                       expected `u32`, found `&mut _`
+   |
+   = note:           expected type `u32`
+           found mutable reference `&mut _`
+help: consider removing `&mut` from the pattern
+   |
+LL -     let _: fn(u32) = |&mut _a| ();
+LL +     let _: fn(u32) = |_a| ();
+   | 
+
+error[E0308]: mismatched types
+  --> $DIR/ref-pat-suggestions.rs:13:25
+   |
+LL |     let _: fn(&u32) = |&&_a| ();
+   |                         ^--
+   |                         ||
+   |                         |expected due to this
+   |                         expected `u32`, found reference
+   |
+   = note:   expected type `u32`
+           found reference `&_`
+help: consider removing `&` from the pattern
+   |
+LL -     let _: fn(&u32) = |&&_a| ();
+LL +     let _: fn(&u32) = |&_a| ();
+   | 
+
+error[E0308]: mismatched types
+  --> $DIR/ref-pat-suggestions.rs:14:33
+   |
+LL |     let _: fn(&mut u32) = |&mut &_a| ();
+   |                                 ^--
+   |                                 ||
+   |                                 |expected due to this
+   |                                 expected `u32`, found reference
+   |
+   = note:   expected type `u32`
+           found reference `&_`
+help: consider removing `&` from the pattern
+   |
+LL -     let _: fn(&mut u32) = |&mut &_a| ();
+LL +     let _: fn(&mut u32) = |&mut _a| ();
+   | 
+
+error[E0308]: mismatched types
+  --> $DIR/ref-pat-suggestions.rs:15:25
+   |
+LL |     let _: fn(&u32) = |&&mut _a| ();
+   |                         ^^^^^--
+   |                         |    |
+   |                         |    expected due to this
+   |                         expected `u32`, found `&mut _`
+   |
+   = note:           expected type `u32`
+           found mutable reference `&mut _`
+help: consider removing `&mut` from the pattern
+   |
+LL -     let _: fn(&u32) = |&&mut _a| ();
+LL +     let _: fn(&u32) = |&_a| ();
+   | 
+
+error[E0308]: mismatched types
+  --> $DIR/ref-pat-suggestions.rs:16:33
+   |
+LL |     let _: fn(&mut u32) = |&mut &mut _a| ();
+   |                                 ^^^^^--
+   |                                 |    |
+   |                                 |    expected due to this
+   |                                 expected `u32`, found `&mut _`
+   |
+   = note:           expected type `u32`
+           found mutable reference `&mut _`
+help: consider removing `&mut` from the pattern
+   |
+LL -     let _: fn(&mut u32) = |&mut &mut _a| ();
+LL +     let _: fn(&mut u32) = |&mut _a| ();
+   | 
+
+error[E0308]: mismatched types
+  --> $DIR/ref-pat-suggestions.rs:18:14
+   |
+LL |     let _ = |&_a: u32| ();
+   |              ^^^  --- expected due to this
+   |              |
+   |              expected `u32`, found reference
+   |
+   = note:   expected type `u32`
+           found reference `&_`
+help: to take parameter by ref, move `&` to the type
+   |
+LL -     let _ = |&_a: u32| ();
+LL +     let _ = |_a: &u32| ();
+   | 
+
+error[E0308]: mismatched types
+  --> $DIR/ref-pat-suggestions.rs:19:14
+   |
+LL |     let _ = |&mut _a: u32| ();
+   |              ^^^^^^^  --- expected due to this
+   |              |
+   |              expected `u32`, found `&mut _`
+   |
+   = note:           expected type `u32`
+           found mutable reference `&mut _`
+help: to take parameter by ref, move `&mut` to the type
+   |
+LL -     let _ = |&mut _a: u32| ();
+LL +     let _ = |_a: &mut u32| ();
+   | 
+
+error[E0308]: mismatched types
+  --> $DIR/ref-pat-suggestions.rs:20:15
+   |
+LL |     let _ = |&&_a: &u32| ();
+   |               ^^^  ---- expected due to this
+   |               |
+   |               expected `u32`, found reference
+   |
+   = note:   expected type `u32`
+           found reference `&_`
+help: consider removing `&` from the pattern
+   |
+LL -     let _ = |&&_a: &u32| ();
+LL +     let _ = |&_a: &u32| ();
+   | 
+
+error[E0308]: mismatched types
+  --> $DIR/ref-pat-suggestions.rs:21:19
+   |
+LL |     let _ = |&mut &_a: &mut u32| ();
+   |                   ^^^  -------- expected due to this
+   |                   |
+   |                   expected `u32`, found reference
+   |
+   = note:   expected type `u32`
+           found reference `&_`
+help: consider removing `&` from the pattern
+   |
+LL -     let _ = |&mut &_a: &mut u32| ();
+LL +     let _ = |&mut _a: &mut u32| ();
+   | 
+
+error[E0308]: mismatched types
+  --> $DIR/ref-pat-suggestions.rs:22:15
+   |
+LL |     let _ = |&&mut _a: &u32| ();
+   |               ^^^^^^^  ---- expected due to this
+   |               |
+   |               expected `u32`, found `&mut _`
+   |
+   = note:           expected type `u32`
+           found mutable reference `&mut _`
+help: consider removing `&mut` from the pattern
+   |
+LL -     let _ = |&&mut _a: &u32| ();
+LL +     let _ = |&_a: &u32| ();
+   | 
+
+error[E0308]: mismatched types
+  --> $DIR/ref-pat-suggestions.rs:23:19
+   |
+LL |     let _ = |&mut &mut _a: &mut u32| ();
+   |                   ^^^^^^^  -------- expected due to this
+   |                   |
+   |                   expected `u32`, found `&mut _`
+   |
+   = note:           expected type `u32`
+           found mutable reference `&mut _`
+help: consider removing `&mut` from the pattern
+   |
+LL -     let _ = |&mut &mut _a: &mut u32| ();
+LL +     let _ = |&mut _a: &mut u32| ();
+   | 
+
+error: aborting due to 18 previous errors
+
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/pattern/for-loop-bad-item.stderr b/src/test/ui/pattern/for-loop-bad-item.stderr
index 9410e4da8d2..886d815d70b 100644
--- a/src/test/ui/pattern/for-loop-bad-item.stderr
+++ b/src/test/ui/pattern/for-loop-bad-item.stderr
@@ -5,10 +5,14 @@ LL |     for ((_, _), (&mut c, _)) in &mut map {
    |                   ^^^^^^         -------- this is an iterator with items of type `(&(char, char), &mut (char, char))`
    |                   |
    |                   expected `char`, found `&mut _`
-   |                   help: you can probably remove the explicit borrow: `c`
    |
    = note:           expected type `char`
            found mutable reference `&mut _`
+help: consider removing `&mut` from the pattern
+   |
+LL -     for ((_, _), (&mut c, _)) in &mut map {
+LL +     for ((_, _), (c, _)) in &mut map {
+   | 
 
 error[E0308]: mismatched types
   --> $DIR/for-loop-bad-item.rs:14:14
diff --git a/src/test/ui/suggestions/match-ergonomics.stderr b/src/test/ui/suggestions/match-ergonomics.stderr
index 4eab2df3080..a9342f9fc30 100644
--- a/src/test/ui/suggestions/match-ergonomics.stderr
+++ b/src/test/ui/suggestions/match-ergonomics.stderr
@@ -4,13 +4,15 @@ error[E0308]: mismatched types
 LL |     match &x[..] {
    |           ------ this expression has type `&[i32]`
 LL |         [&v] => {},
-   |          ^^
-   |          |
-   |          expected `i32`, found reference
-   |          help: you can probably remove the explicit borrow: `v`
+   |          ^^ expected `i32`, found reference
    |
    = note:   expected type `i32`
            found reference `&_`
+help: consider removing `&` from the pattern
+   |
+LL -         [&v] => {},
+LL +         [v] => {},
+   | 
 
 error[E0529]: expected an array or slice, found `Vec<i32>`
   --> $DIR/match-ergonomics.rs:8:9
@@ -34,13 +36,15 @@ error[E0308]: mismatched types
 LL |     match y {
    |           - this expression has type `i32`
 LL |         &v => {},
-   |         ^^
-   |         |
-   |         expected `i32`, found reference
-   |         help: you can probably remove the explicit borrow: `v`
+   |         ^^ expected `i32`, found reference
    |
    = note:   expected type `i32`
            found reference `&_`
+help: consider removing `&` from the pattern
+   |
+LL -         &v => {},
+LL +         v => {},
+   | 
 
 error[E0308]: mismatched types
   --> $DIR/match-ergonomics.rs:40:13
@@ -49,10 +53,14 @@ LL |     if let [&v] = &x[..] {}
    |             ^^    ------ this expression has type `&[i32]`
    |             |
    |             expected `i32`, found reference
-   |             help: you can probably remove the explicit borrow: `v`
    |
    = note:   expected type `i32`
            found reference `&_`
+help: consider removing `&` from the pattern
+   |
+LL -     if let [&v] = &x[..] {}
+LL +     if let [v] = &x[..] {}
+   | 
 
 error: aborting due to 5 previous errors